Use Python 3.8 in AzDo pipelines

[kimadeline]

For #8298

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR).
• Title summarizes what is changing.
• Has a news entry file (remember to thank yourself!).
• Appropriate comments and documentation strings in the code.
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated.
• Test plan is updated as appropriate.
• package-lock.json has been regenerated by running npm install (if dependencies have changed).
• The wiki is updated with any design decisions/details.

[codecov-io]

Codecov Report

Merging #10276 into master will decrease coverage by 0.44%.
The diff coverage is 49.39%.

@@            Coverage Diff             @@
##           master   #10276      +/-   ##
==========================================
- Coverage   61.27%   60.82%   -0.45%     
==========================================
  Files         570      570              
  Lines       30958    30720     -238     
  Branches     4441     4376      -65     
==========================================
- Hits        18969    18685     -284     
- Misses      11001    11082      +81     
+ Partials      988      953      -35

Impacted Files	Coverage Δ
...c/datascience-ui/react-common/settingsReactSide.ts	22.22% <ø> (ø)	⬆️
.../client/testing/nosetest/services/parserService.ts	12.34% <ø> (ø)	⬆️
src/client/testing/pytest/services/argsService.ts	39.21% <ø> (ø)	⬆️
src/client/telemetry/index.ts	87.38% <ø> (ø)	⬆️
src/datascience-ui/react-common/constants.ts	0% <0%> (ø)	⬆️
.../datascience/interactive-common/interactiveBase.ts	5.72% <0%> (-10.92%)	⬇️
...ractive-window/interactiveWindowCommandListener.ts	58.09% <0%> (ø)	⬆️
src/client/datascience/webViewHost.ts	8.78% <0%> (-51.77%)	⬇️
...atascience/interactive-window/interactiveWindow.ts	16.44% <10%> (-0.92%)	⬇️
src/client/common/application/types.ts	100% <100%> (ø)	⬆️
... and 40 more

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
100.0% Duplication